staging: hv: check return value of RingBufferInit()

RingBufferInit() would always return sucess and instead relied on an
ASSERT() to test for an error condition.  Remove the ASSERT() and
return -EINVAL instead.  The return value of RingBufferInit() was also
never checked, so check it.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>